1.00 Tutorial 11
HashTables
Goals of this Tutorial
• Understand utility methods
– equals(), toString(), hashCode()
• Understand HashTables
– what a HashTable is
– how to use a HashTable
• Understand hash codes
– what is a hash code?
– what makes a good hash code?
The Object Class
• Object class methods:
public boolean equals (Object obj)
Indicates whether another
object is equal to this one.
public String toString()
Returns a string
representation of the object.
public int hashCode()
Returns a hash code value
for the object.
The equals() Method
public class Student {
int id;
public Student(int id){
this.id = id;
}
public static void main(String args[]){
Student s1 = new Student(12); // student id is 12
Student s2 = new Student(12); // student id is also 12
Student s3 = s1;
// reference same object
s1.equals(s2); // what will default equals() method return?
s1.equals(s3); // what will default equals() method return?
}
}
QUESTION: how do you make equals() return true if two students
objects have the same ID? (Hint: override the default equals()
method.)
The equals() Method (cont’d)
public class Student {
public Student(int id){
this.id = id;
}
public boolean equals (Object obj) {
if (obj != null && obj instanceof Student)
return id == ((Student) obj).id;
else
return false;
}
public static void main(String args[ ]){
Student s1 = new Student(12); // student id is 12
Student s2 = new Student(12); // student id is also 12
Student s3 = s1;
s1.equals(s2); // what does it return now?
s1.equals(s3); // what does it return now?
}
}
The toString() Method
public class Student {
int id;
public Student(int id){
this.id = id;
}
//we override the default toString() method
public String toString()
{ return new String ("ID: " + id);}
public static void main(String args[]) {
Student s = new Student(12);
System.out.println(s);
// ID: 12
System.out.println ("Student " + s); //
“Student ID: 12”
System.out.println (s.toString()); // ID: 12
}
}
hashCode() method
• hashCode(): returns reasonably uniform
distribution of integers based on object
data
• equals(): returns true if the objects’ data
are equal.
Note: if two objects a and b are equal
(a.equals(b) returns true), then their hashCodes must have the same value.
Recollection of our collections
elem[0]
elem[1]
item 1
elem[2]
item 2
item 3
linked list
...
array
binary
search
tree
Pop quiz: what is the average search
time for each of these? Worst case
search time? We want to improve
the average search time.
HashTable
Bucket index is the
hash code of the key
bucket [0]
Contents of a bucket are a collection
of objects whose keys hash to the same value
(0 in this case). Can be a linked list, array,
Vector, binary tree, etc.
Obj 1
Obj 2
null
bucket [1]
bucket [2]
…
bucket [n]
Obj 2: key
Obj 2: value
In general,
objects in the
hash table have
two components:
a key, and a
value. The hash
code is computed
on the key. The
value is the data
that is stored. In
some cases, the
key and value will
be the same.
Super Simple Example
• We will design hash table with the
following restrictions
– Key is the same as the value
– Can only hold Strings
– Hash function is the length of the String
– Always has just 5 buckets
Let's go through the file
SuperSimpleHashTable.java
hashCode()
• In reality, it is often very difficult to come
up with a good hash code algorithm
• hash codes should be evenly distributed
across the range of integers
• if we are using the modulo operator (%) to
find the right bucket, the hash table should
use a prime number of buckets
hashCode (2)
• What was wrong with our hash code
algorithm in the SuperSimpleHashTable?
• Why was it okay?
• Let's think about a hash code algorithm for
the Person class used in PS#9, and
PS#10
Person (PS9 & 10)
public class Person {
String name;
int[] myProfile;
// me
int[] yourProfile; // desired
public int hashCode() {
/* ? */
}
public boolean equals(Object o) { /* ? */
} }
You don't necessarily need these methods for PS#10;
we just use the Person class as an example.
Hash Table Questions
1. What would be the result if hashCode()
returned the same value for all objects?
2. How many objects can a hash table
hold?
3. What is the effect of adding objects to a
hash table if they are sorted vs. in
random order?
4. What can you say about a hash table
that has n buckets and 10n objects?
Answers
1. All objects would get stored in the same
bucket, making the hash table useless.
2. In theory, there is no limit. (In practice,
limited by memory and hard disk space.)
3. No effect. Hash tables do not store
objects in order.
4. It is very inefficient; there will necessarily
be many hash collisions. Load factor (#
objects/#buckets) should be between 0.5
and 1.